Oscillator circuits are commonly used in electronic circuit designs to generate an oscillating output signal. These oscillator circuits may be found in clock generation circuits, phase-locked loop (PLL) circuits, timing circuits, and so on. For example, a ring oscillator, which generally includes an odd number of inverters connected in series, may be used as part of a PLL for clock and data recovery, frequency synthesis, and clock synchronization.
Differential oscillators are commonly used in order to suppress noise. For example, the differential oscillator may be based on a first Colpitts oscillator and a mirror image Colpitts oscillator that is coupled to the first Colpitts oscillator. The differential oscillator outputs differential output signals that are about 180 degrees out of phase. Colpitts oscillators are well-known in the art as are other differential oscillator designs.
With a differential scheme, any element in an electronic system that has to suppress noise is split into two branches. One of the branches contains the normal information, whereas the other branch contains the complementary (inverted) information. With proper matching of components, a highly symmetric design between the two branches can usually be obtained. Since the useful information is recovered or extracted from the difference in state between the two branches, common mode disturbances that affect both branches equally are ideally balanced out completely. Differential output stages are used in many conventional oscillator circuits and other circuit designs to provide balanced transmission properties with good noise immunity.
An oscillator circuit or other circuit providing a differential output may be used in applications in which the circuit is exposed to harsh environments, such as space and military applications. However, these circuits may be susceptible to Single Event Effects (SEE). SEE is a disturbance in an active semiconductor device caused by a single energetic particle. As semiconductor devices become smaller and smaller, transistor threshold voltages decrease. These lower thresholds reduce the charge per node needed to cause errors. As a result, the semiconductor devices become more and more susceptible to transient upsets.
One type of SEE is a single event upset (SEU). SEU is a radiation-induced error in a semiconductor device caused when charged particles lose energy by ionizing the medium through which they pass, leaving behind a wake of electron-hole pairs. The electron-hole pairs form a parasitic conduction path, which can cause a false transition on a node. The false transition, or glitch, can propagate through the semiconductor device and may ultimately result in the disturbance of a node containing state information, such as an output of a latch, register, or gate.
Typically, an SEU is caused by ionizing radiation components, such as neutrons, protons, and heavy ions. The ionizing radiation components are abundant in space and at commercial flight altitudes. Additionally, an SEU may be caused by alpha particles from the decay of trace concentrations of uranium and thorium present in some integrated circuit packaging. As another example, an SEU may be caused by detonating nuclear weapons. When a nuclear weapon is detonated, intense fluxes of gamma rays, x-rays, and other high energy particles are created, which may cause SEU.
For example, transistors in an oscillator circuit may be susceptible to SEU. As a result, the oscillator circuit may not provide a periodic output that can be reliably used as a clock signal or for other purposes. While others have used triple modular redundancy or other voting schemes to harden oscillator circuits against the effects of SEU, providing three oscillator circuits is typically prohibitive due to space and process variances.
Thus, it would be beneficial to harden an oscillator circuit or other differential output circuit so that these circuits may be used in applications that are susceptible to SEE without having to triplicate the circuit in a circuit design.